So when you took the circuit board out, you added the LED on the board so you can test it while it was removed from the car so you did not have to go back and forth? Then when you are happy with the test result, you but the LED back into the clown nose? Just want to have this clarified.

I did not take my rear view mirror apart so I do not know how the original circuit board look like. I cannot tell from looking at your photo which wires you added to the board.

Also, how can you be so sure that your DIY did not create any kind of current leak? I guess this is where the MS in EEE helps.